A multiplate wet clutch for an automatic transmission has an alternate arrangement of multiple friction plates each with a friction material formed of a special sheet of paper attached to a surface thereof and separator plates to contact the friction plates. Action of making a switch between separating and connecting the friction plates and the separator plates controls transmission of power.
The friction plate and the separator plate are both ring-shaped steel plate members. Generally, such a friction plate and a separator plate forming the multiplate wet clutch are collectively called a clutch plate.
The following four phenomena are known as major defect phenomena occurring in the separator plate: wear of a spline part (hereinafter called feature A); a rattle due to an inaccurate position of the spline part (hereinafter called feature B); change in roughness due to wear of a surface in friction with the friction plate (hereinafter called feature C); and the occurrence of a heat spot and nonuniformity of a shape and a material quality due to the heat spot (hereinafter called feature D). All of these phenomena are significant characteristics, among which a defect due to the heat spot is the hardest to deal with.
In response to behavior of the multiplate wet clutch to make a shift from a neutral state to a power transmission state to engage the clutch, the friction plate and the separator plate are pressed against each other under high load and a high relative velocity. This rapidly reduces the relative velocity between the friction plate and the separator plate. Resultant frictional heat rapidly enters a surface of the separator plate to become a sliding portion, thereby increasing the temperature of the surface of the separator plate. This temperature increase in the surface of the separator plate becomes a cause for the occurrence of a heat spot.
A projection at the heat spot part resulting from heating with the frictional heat, distortion occurring around the heat spot, and local change in a material quality cause a nonuniform frictional state when the clutch is actuated. The nonuniform frictional state causes a new heat spot. Such a vicious cycle degrades the performance of the multiplate wet clutch to a large extent.
Enhancing fuel efficiency of automobiles is an extremely important problem to be solved at the present time. Enhancing efficiency in terms of a mechanistic aspect and reducing the size and weight of the clutch as a unit are very important elements among various elements forming an automobile.
Requirements for a transmission include enhancement of efficiency, reduction in friction loss, and reduction in size and weight. The efficiency of the multiplate wet clutch should be enhanced to satisfy these requirements. The efficiency of the multiplate wet clutch may be enhanced for example by reduction in the diameter of a plate, reduction in the number of plates, and increase in a coefficient of friction achieved by reducing a lubricant and changing a friction material. However, all of these become causes for excessive temperature increase, specifically, a heat spot that cannot be handled with a conventional technique.
Enhancing the performance of the clutch plate in terms of its material may lead to dramatic enhancement of the efficiency of the transmission. Thus, enhancing heat spot resistance is required for a steel plate to be used as the clutch plate.
Methods described, for example, in PTLs 1 to 5 are known as techniques relating to enhancement of the heat spot resistance of a steel plate.
According to the method described in PTL 1, temperature of phase transformation from ferrite to austenite is increased using low-carbon steel. This prevents the occurrence of phase transformation even if a plate is heated by frictional heat during engagement of a clutch, thereby suppressing the occurrence of a heat spot.
According to the method described in PTL 2, the thermal diffusivity of a steel plate is increased by defining an alloy element content. This suppresses temperature increase of a plate to be caused by frictional heat to suppress the occurrence of a heat spot.
According to the method described in PTL 3, austenitic stainless steel unlikely to be phase transformed is used as a material for a plate, thereby suppressing the occurrence of a heat spot.
According to the method described in PTL 4, a Ti precipitate or an Nb precipitate is used to suppress the occurrence of a heat spot.
According to the method described in PTL 5, in addition to using a Ti precipitate or an Nb precipitate, Si or Al having the effect of increasing a transformation point is added to suppress the occurrence of a heat spot.
In addition to enhancement of efficiency, reduction in friction loss, and reduction in size and weight of a transmission achieved by enhancing heat spot resistance, the antiwear performance of a spline part such as a tooth tip of the separator plate is also one important characteristic.
Methods described in PTLs 6 to 9 are known as techniques relating to enhancement of the antiwear performance of a tooth tip of the separator plate.
According to the method described in PTL 6, antiwear performance is enhanced by using a hard precipitate such as TiC or cementite.
According to the method described in PTL 7, a hot-rolled steel plate having a ferrite structure with ferrite particles with a diameter of 5 μm or more and 15 μm or less is cold rolled at rolling reduction of 50% or more, thereby enhancing antiwear performance.
According to the method described in PTL 8, a steel structure is controlled through combined addition of Cr, Ti, and B, thereby enhancing antiwear performance.
According to the method described in PTL 9, a steel structure is controlled by controlling a fraction of pearlite and that of cementite and controlling the diameter of ferrite particles, thereby enhancing antiwear performance.